/* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol
除了您自己定义的symbols之外，JavaScript还有一些内置的symbols，这些符号代表了在ECMAScript 5和之前没有向开发人员公开的内部语言行为。可以使用以下属性访问这些symbols

In addition to your own symbols, JavaScript has some built-in symbols which represent internal language behaviors which were not exposed to developers in ECMAScript 5 and before. These symbols can be accessed using the following properties:
*/
